The release of certain chemical species such as SO.sub.2, CO.sub.2, NO.sub.x and volatile organic chemicals into the air and atmosphere has become regulated. Coal burning power plants in particular have been searching for ways to decrease the levels of SO.sub.2 which escapes from the plant and into the atmosphere. In attempts to comply with environmental regulations various scrubbing systems have been developed. In each system the SO.sub.2 gas is passed over or through the scrubbing medium to generate an acidified sulfur rich medium. Scrubbing may be accomplished via lime and limestone scrubbing, magnesium oxide scrubbing or sodium scrubbing with thermal, electrolytic or electrodialytic regeneration. The present invention is directed toward an improved process which utilizes aqueous scrubbing followed by electrodialytic regeneration.
U.S. Pat. Nos. 4,082,835 and 4,107,015 disclose absorbing SO.sub.2 into a basic compound to achieve a substantially soluble, acidic compound followed by regeneration of the basic compound and liberation of the SO.sub.2 via treatment in a two compartment electrodialytic water splitter. The product from the acid compartment of the water splitter may be stripped to remove the SO.sub.2 and the remaining acid product may be treated in a three compartment water splitter to recover acid and base which may be recycled to the scrubbing stage.
U.S. Pat. No. 4,552,635 discloses a method for removing and recovering sulfur oxides from gases and regenerating the process liquors by contacting the gases with an aqueous hydroxide in a first reaction zone to produce an aqueous salt solution, contacting the aqueous salt solution with aqueous sulfuric acid in a second reaction zone to form sulfurous acid and aqueous soluble sulfates, removing the sulfurous acid by liberating gaseous SO.sub.2, and regenerating the sulfuric acid and hydroxide solution via electrodialytic treatment. Either a two or three compartment water splitter may be used and the sulfuric acid and aqueous hydroxide which are recovered may be recycled back to the appropriate reaction zones.
U.S. Pat. No. 4,592,817 discloses an improved electrodialytic water splitting process for recovering metal or ammonium values from materials comprising a salt of a first acid while avoiding formation of gas bubbles in the electrodialytic unit. The salt of a first acid is contacted with a solution comprising a second acid to produce a solution containing the first acid and a salt of the second acid. The first acid is recovered and the second acid is regenerated via electrodialytic treatment in either a two or three compartment water splitter.
U.S. Pat. No. 4,629,545 discloses an electrodialytic water splitting process for recovering SO.sub.2 -containing gases from the alkaline sodium scrubbing of sulfur dioxide from SO.sub.2 -containing gases and converting the spent scrubbing materials into sodium hydroxide. The disclosed process includes contacting SO.sub.2 -containing gas with an aqueous solution containing Na.sub.2 CO.sub.3, NaHCO.sub.3 or mixtures thereof, recovering the aqueous reaction mixture, and treating the aqueous reaction mixture in an electrodialytic water splitter to produce a H.sub.2 SO.sub.3 solution from which gaseous SO.sub.2 may be produced, and an aqueous hydroxide ion-enriched solution. Either a two or three compartment water splitter may be used.
However, the processes described above all suffer from practical drawbacks which result in a decrease in process efficiency. Flyash particulates in the flue gas are solubilized in the SO.sub.2 absorption step. Once the solubilized particulates are fed into the electrodialytic water splitter metals tend to precipitate inside the base loop of the two compartment cell, and on the cation and bipolar membranes. Typical metal contaminants in the absorber liquid include Ca, Mg, Fe, V, Si, Al, Cr and Ni. These elements have sufficiently low solubilities at the higher pH which is typically found in the base loop and at the anion surfaces of the bipolar membranes that the metals precipitate and foul the membranes and other internal features of the cell stack such as gaskets, membrane supports and spacers. Calcium, magnesium and iron are frequently present in quantities between about 10 ppm and 250 ppm. One option is to pretreat the feed solution to remove the multivalent metals by, for example, using chelating resins or ion exchange. However, pretreatment steps are not preferred because they add to the complexity and cost of the process. Attempts to control the pH of the base loop at a level above which the metal contaminants are soluble to mitigate fouling have met with only limited success. Further, metals in the acid loop transport across the cation membranes to the base loop and increase the concentration of contaminants, thereby exacerbating the fouling problem. Attempts to reduce fouling by increasing the circulation rates and adding periodic wash steps help abate the fouling problem, but are not a totally satisfactory solution. Thus, there remains a continuing need for a practical solution to the fouling problems experienced in the electrodialytic processing of SO.sub.2 -containing gases. A goal of the present invention is to minimize either the amount of metal ions which are fed into the electrodialytic water splitter or their effect on the membranes.
Another problem is the precipitation of sulfite and sulfate in anion layer of the bipolar membrane adjacent to the base loop due to the high ionic strength of the solutions which are processed there. Sulfite/sulfate precipitation damages the bipolar membranes. It is desirable to operate the absorber at the highest feed solution concentration possible. However, high concentrations of Na.sup.+ are transported from the acid loop to the base loop generating NAOH and further increasing the ionic strength and sulfite/sulfate precipitation. Thus, another object of the present invention is to decrease the ionic strength of the solution treated in the base loop without decreasing the spent absorbent concentration.